High-pressure metal-vapor discharge tube

ABSTRACT

In a discharge tube of saturated metal vapor pressure type containing high-pressure gaseous or vaporized metals, the temperature of the coolest points existing at both ends of the discharge tube can be raised by forming a layer or layers of a metal or metals of high melting point, low vapor pressure and good thermal conductivity, such as niobium, on the outer wall at the ends of the discharge tube; thereby, providing better radiant emission, especially higher color temperature and an improved color rendering property compared with conventional high-pressure metal-vapor discharge tubes. The discharge tube of the abovementioned construction can be manufactured easily and has a longer life under burning conditions corresponding to those for conventional type high-pressure metal-vapor discharge tubes.

United States Patent 1 1 Mizuno et al.

[ 51 Feb. 13, 1973 [54] HIGH-PRESSURE METAL-VAPOR DISCHARGE TUBE [75]Inventors: llideo Mizuno; Sadao Kimura, both [73] Assignee: MatsushitaElectronics Corporation, Kadoma City, Osaka Prefecture, Japan 22 Filed:Aug. 28, 1970 21 Appl. No.: 67,691

[30] Foreign Application Priority Data [58] Field of Search ..3l3/47,220, 221, 318, 317

[5 6] References Cited UNITED STATES PATENTS Lange ..3l3/3l7 UX3,473,071 10/1969 Rigdcn ct a1. ..313/22O 2,987,813 6/1961 Pope et ul.313/317 UX 3,497,756 2/1970 Knochcl cl ul i ..3l3/22() X 3,450,9246/1969 Knochcl ct a1 .313/221 X Primary ExaminerAlfred L. BrodyAttorneyCraig, Antonelli and Hill [57] ABSTRACT In a discharge tube ofsaturated metal vapor pressure type containing high-pressure gaseous orvaporized metals, the temperature of the coolest points existing at bothends of the discharge tube can be raised by forming a layer or layers ofa metal or metals of high melting point, low vapor pressure and goodthermal conductivity, such as niobium, on the outer wall at the ends ofthe discharge tube; thereby, providing better radiant emission,especially higher color temperature and an improved color renderingproperty compared with conventional high-pressure metal-vapor dischargetubes. The discharge tube of the above-mentioned construction can bemanufactured easily and has a longer life under burning conditionscorresponding to those for conventional type high-pressure metal-vapordischarge tubes.

10 Claims, 3 Drawing Figures HIGH-PRESSURE METAL-VAPOR DISCHARGE TUBEBACKGROUND OF THE INVENTION This invention relates to a high-pressuremetal-vapor discharge tube consisting of a sealed tubular enclosure of ahigh-melting transparent polycrystalline ceramic, a pair of electrodes,each of which is enclosed near respective ends of said enclosure, and asmall amount of metals, which are converted to the gaseous state whenthe tube is in operation.

High-pressure metal-vapor discharge tubes can be classified into twotypes; namely, the unsaturated type wherein confined metals, such assodium, are completely vaporized to the gaseous state when the tube isin operation, and the saturated type wherein confined metals are notcompletely vaporized thereby retaining a part of the metals in the solidor liquid state. In the latter type, the saturated type discharge tube,said solid or liquid metals remain at the coolest points at both ends ofthe enclosure. Generally, the radiant emission characteristics of thedischarge tube, such as color temperature and the color renderingproperty, depend on, and will vary with, the pressure of the vaporizedmetals which is determined by the temperature of the coolest points inthe tube.

In an example of conventional high-pressure sodium lamps, thetemperature of the coolest points are designed to be in the range around600 to 700 C to control the sodium vapor pressure within the rangearound 100 to 200 torr; such high-pressure sodium lamps can only attaina color temperature of 2,l K and a color rendering index of 25, whichconditions are not quite satisfactory for general lighting applications.Any sodium vapor pressure in the range of 300 to 1,000 torr can improvethe radiant emission characteristics of such lamps, especially the colorrendering property.

To obtain a sodium vapor pressure higher than 300 torr, it is necessaryto raise the temperature of the coolest points at both ends of thedischarge tube. There has been a proposal to provide a thermalinsulation coating at the coolest points of the discharge tubes formetal-halide lamps and a high-pressure mercury vapor lamps. The examplesof these prior art devices employ a thermal insulating coating, such astitanium oxide or carbon, at both ends around the sealing portion of theouter surface of the discharge tube. These prior art devices attain athermal insulating effect to raise the temperature of the coolestpoints, but the lack of thermal conductivity of the coating material isnot suitable for obtaining an ideal heat distribution over the entirelength of the tubular enclosure.

SUMMARY OF THIS INVENTION The primary object of this invention is toobtain a high-pressure metal-vapor discharge tube with improved colortemperature and a good color rendering property of radiated emission,which is desirable for general lighting applications. To obtain improvedcolor temperature and a good color rendering property, the sodium vaporpressure of such a discharge tube must be higher than 300 torr; and, thesimplest way to increase the sodium vapor pressure is to apply a largerlamp input power, which causes undesirable increase in wall loading ofthe discharge tube and results in thermal decomposition of the aluminaceramic tubular enclosure, if the temperature of tubular enclosureexceeds 1 ,200 C.

The inventors have discovered that the temperature of the coolest pointsof a high-pressure metal-vapor discharge tube can be raised by providinga layer or layers of thermally conductive metal or metals having a highmelting point, low vapor pressure and good thermal conductivity, bywinding a metal foil or foils tightly, by chemical or vacuum depositionor by sputtering a metal or metals directly, around the coolest pointslocated at both ends of the discharge tube of a sealed tubular enclosureof transparent polycrystalline ceramics, such as a high-density alumina,wherein a small amount of mercury and alkali metal are confined, therebyeffectively insulating the thermal radiation from the coolest points andconducting the heat from the central part of tubular enclosure to thecoolest points.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages will bebest understood from the following detailed description when read inconjunction with the accompanying drawings, in which:

FIG. 1 is a sectional side view of a high-pressure metal-vapor dischargetube embodying this invention,

FIG. 2 is a graph indicating the outer wall temperature distribution inthe longitudinal direction, and

FIG. 3 is a graph indicating the spectral distribution of the dischargetube of the above embodiment.

DETAILED DESCRIPTION In FIG. 1, each end part of a discharge tubeenclosure 1, which consists of a transparent polycrystalline ceramictube, such as high-density alumina, is hermetically sealed at both endsby respective end discs 2,,and each lead-in metal tube 3 hermeticallypenetrates through one end disc 2.

The surfaces between said discharge tube enclosure 1 and each end disc 2and those between each lead-in metal tube 3 and each end disc 2 aresealed hermetically with ceramic cement, and in said tubular enclosure astarting rare gas, such as xenon gas, and a substantial quantity of ametal of a discharging medium 4, for instance, sodium, together withmercury, which serves as a buffer gas, are confined. As a material forthese end discs 2, the same ceramic material as that of said tubularenclosure 1 is preferred, but such ceramics whose thermal expansioncoefficient approximates that of the lead-in tubes 3 of a metal of highmelting point, such as niobium, tantalum, molybdenum, etc., can be used.Also a metal end disk of a high melting metal as indicated can be used.On both end parts of the tubular enclosure, around their outer wall,thermally conductive layers 5, which characterize this invention, areprovided. For such thermally conductive layers 5, a metal having a highmelting point and low vapor pressure and good thermal conductivity,selected from a group consisting of titanium, vanadium, rhodium,ruthenium, molybdenum, niobium, tantalum, tungsten, platinum,

' iridium, rhenium and osmium, can be used.

Such thermally conductive layers 5 are formed by winding a metal foil orfoils tightly, by chemical or vacuum deposition or by a sputteringmethod. Said thermally conductive layers 5 function to raise thetemperature of the coolest points at both ends of the tubular enclosure.

Namely, as a general rule of this kind of discharge tube, the dischargetube 1 is a straight tube of 6 to 15 mm inner diameter. While inoperation, its central outer wall surface is heated to about l,200 C,from which area the outer wall surface temperature decreases toward theends of the tube in a curve as shown in FIG. 2. Owing to said thermallyconductive layers 5 provided on the coolest points located furthertoward the ends than discharge electrodes 6, positioned near both endsin the tubular enclosure 1 on the tips of the lead-in tubes 3, thetemperature of .said coolest points indicated by the solid line in FIG.2 is raised by 50 C or more, as compared with the conventional dischargetube indicated by the dotted line in FIG. 2. This is as a result of thefollowing functions.

1. Heat accumulation on the outer wall surface at the high temperatureportion between the pair of electrodes 6 is efficiently conducted to thetube end portions.

2. Thermal radiation generated inside the tube, especially at the innerend portions of the tube, is reflected inward and confined in each innerend portion without radiating outward, resulting in a rise in thetemperature at the coolest points. Accordingly, a temperature of 650 to800 C at the coolest point can be obtained while keeping the highesttemperature at the central outer wall surface of the tube at l,200 C.

The use of metal end caps, instead of said ceramic end discs 2 and 2produces the same effect in this invention.

According to the experiments of the inventors of this invention, apractical discharge tube of high color rendering property has beenobtained by providing eachof said thermally conductive layers 5 aroundthe outer wall surface of the tube. Such thermally conduc tive layers 5on the outer wall surface are preferably limited within an area betweenthe ends of the tubular enclosure and 5 mm from the front tip 7 of eachelectrode 6 toward the center of the enclosure 1 at both ends of tube.

In an example, when a tantalum foil of 0.02 mm thickness with a width of12 mm is wound tightly around the tubular enclosure at both ends of thetube between the position 5 mm from the front tip of the electrodetoward the central part and the tube end, a spectral distribution shownin FIG. 3 was obtained at a tube wall loading of 20 watts/cm. Lampcharacteristics of this discharge tube are summarized as a lamp voltageof 320 volts, color temperature of 3,000 K and color rendering index of78 per the C.I.E. (Commission Internationale de lEclairage)recommendation. In the conventional high-pressure sodium discharge lampfor the same input power rating as this example, the lamp voltage of 100volts, color temperature of 2,l K and color rendering index of 25 perthe C.I.E. recommendation were attained. Therefore, the embodiment ofthis invention has an outstanding superiority to said prior art devicewith respect to the color rendering property.

In the present example, if the thermally conductive layers are extendedbeyond said limit of 5 mm toward the center of the enclosure fromthe'front tip of the electrode 6, it would shield 21 part of the radiantemission, reducing the luminous efficacy of the discharge tube.Therefore, for practical purposes, the layers should be limited, asmentioned above, between the further-most end of the tube and 5 mm fromthe front tip of the electrode 6 toward the center of the enclosure 1.

For the thermally conductive layers 5, any metal selected from theaforementioned group can be used in place of tantalum. As an example, inthe case of applying a molybdenum film by chemical deposition, the endpart of a ceramic tube is preparatorily heated in 600 to 700 C, and amixed gas of molybdenum pentachloride (MOCL5) and hydrogen is made tocontact the desired parts of the tube, whereon molybdenum is depositedto form films. Such a tube can be employed as a discharge tube.

For making a titanium film by vacuum deposition, titanium is evaporatedby heating at, for instance, about 2,000 C, and is deposited for makingthe film layers on the necessary parts of a required ceramic tube.

In case of depositing niobium or tantalum by a sputtering method, bothend parts of the tube are wound with foils of niobium or tantalum, andthe central part of the tube is covered with an electric insulatingsubstance such as a porcelain tube, on which a tungsten positiveelectrode is provided, and discharging is made in the vacuum using saidniobium or tantalum foils as a negative electrode so as to sputter anddeposit said niobium or tantalum onto the tube. As for the metal, any ofthe aforementioned group can be applied in the manner of the saidexamples.

As the material of the tubular enclosure for the discharge tube of thisinvention, transparent polycrystalline high-density ceramics, such ashighdensity alumina, beryllia or magnesia, which are chemically stableagainst sodium vapor and have a high melting point, can be used.

As fully described above, according to this invention, a high-pressuremetal-vapor discharge tube having a high color rendering property isobtainable by an easily practicable method, such as providing thermallyconductive layers constituted with a metal or metals having high meltingpoint and low vapor pressure and good thermal conductivity around bothends of the tubular enclosure, thus enabling attainment of greatindustrial and practical efficiency.

While we have shown and described several embodiments in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to a person skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

We claim:

1. High-pressure metal-vapor discharge tube comprising a sealed tubularenclosure of transparent polycrystalline high-density ceramic, a pair ofelec trodes having rear tip and front tip portions, each of saidelectrodes being. enclosed near a respective end of said enclosure, astarting rare gas, and a small amount of mercury plus at least oneionizable medium of alkali metal sufficient to form a saturated metalvapor confined in said enclosure, and characterized in that:

layers of thermally conductive, heat durable metal are formed on theouter wall of the tubular enclosure proximate to the ends thereof, saidlayers extending from the end of the tubular enclosure beyond the reartip of the electrode to a position of 5 no more than 5 mm from the fronttip of the electrode toward the central part of the tubular enclosure.

2. High-pressure metal-vapor discharge tube of claim 1', wherein saidthermally conductive metal is a metal selected from a group consistingof titanium, vanadium, rhodium, ruthenium, molybdenum, niobium,tantalum, tungsten, platinum, iridium, rhenium and osmium.

3. High-pressure metal-vapor discharge tube of claim 2, wherein eachsaid layer of thermally conductive metal is in the form of a metal foilwound around each end part of the tubular enclosure.

4. High-pressure metal-vapor discharge tube of claim 2, wherein saidlayers are in the form of a chemically decomposed chemical compound ofsaid thermally conductive metal.

5. High-pressure metal-vapor discharge tube of claim 2, wherein saidlayers are in the form of a vacuum deposited thermally conductive metal.

6. High-pressure metal-vapor discharge tube of claim 2, wherein saidlayers are in the form of sputtered thermally conductive metal.

7. High-pressure metal-vapor discharge tube of claim 1, wherein a pairof end discs is hermetically sealed in the end-s of said tubularenclosure, respectively, said pair of electrodes being carried by saidend discs, said end discs being formed from a heat durable metal.

8. High-pressure metal-vapor discharge tube of claim 7, wherein said enddiscs are formed from a material selected from the group consisting ofniobium, tantalum, and molybdenum.

9. High-pressure metal-vapor discharge tube comprising a sealed tubularenclosure of transparent polycrystalline high-density ceramic, a pair ofelectrodes, each of which is enclosed near a respective end of saidenclosure, a starting rare gas, a small amount of mercury plus at leastone ionizable medium of alkali metal sufficient to form a saturatedmetal vapor confined in said enclosure, and at least one layer of athermally conductive heat durable metal formed on the outer wall of thetubular enclosure proximate to the ends thereof, each layer being in theform of a metal foil wound around the tubular enclosure, said at leastone layer extending from the end of the tubular enclosure beyond the tipof the electrode a distance of no more than 5 mm from the front tip ofthe electrode toward the central part of the tubular enclosure.

10. High-pressure metal-vapor discharge tube of claim 9, wherein a pairof ceramic end disks is hermetically sealed in the ends of said tubularenclosure.

1. High-pressure metal-vapor discharge tube comprising a sealed tubular enclosure of transparent polycrystalline high-density ceramic, a pair of electrodes having rear tip and front tip portions, each of said electrodes being enclosed near a respective end of said enclosure, a starting rare gas, and a small amount of mercury plus at least one ionizable medium of alkali metal sufficient to form a saturated metal vapor confined in said enclosure, and characterized in that: layers of thermally conductive, heat durable metal are formed on the outer wall of the tubular enclosure proximate to the ends thereof, said layers extending from the end of the tubular enclosure beyond the rear tip of the electrode to a position of no more than 5 mm from the front tip of the electrode toward the central part of the tubular enclosure.
 2. High-pressure metal-vapor discharge tube of claim 1, wherein said thermally conductive metal is a metal selected from a group consisting of titanium, vanadium, rhodium, ruthenium, molybdenum, niobium, tantalum, tungsten, platinum, iridium, rhenium and osmium.
 3. High-pressure metal-vapor discharge tube of claim 2, wherein each said layer of thermally conductive metal is in the form of a metal foil wound around each end part of the tubular enclosure.
 4. High-pressure metal-vapor discharge tube of claim 2, wherein said layers are in the form of a chemically decomposed chemical compound of said thermally conductive metal.
 5. High-pressure metal-vapor discharge tube of claim 2, wherein said layers are in the form of a vacuum deposited thermally conductive metal.
 6. High-pressure metal-vapor Discharge tube of claim 2, wherein said layers are in the form of sputtered thermally conductive metal.
 7. High-pressure metal-vapor discharge tube of claim 1, wherein a pair of end discs is hermetically sealed in the ends of said tubular enclosure, respectively, said pair of electrodes being carried by said end discs, said end discs being formed from a heat durable metal.
 8. High-pressure metal-vapor discharge tube of claim 7, wherein said end discs are formed from a material selected from the group consisting of niobium, tantalum, and molybdenum.
 9. High-pressure metal-vapor discharge tube comprising a sealed tubular enclosure of transparent polycrystalline high-density ceramic, a pair of electrodes, each of which is enclosed near a respective end of said enclosure, a starting rare gas, a small amount of mercury plus at least one ionizable medium of alkali metal sufficient to form a saturated metal vapor confined in said enclosure, and at least one layer of a thermally conductive heat durable metal formed on the outer wall of the tubular enclosure proximate to the ends thereof, each layer being in the form of a metal foil wound around the tubular enclosure, said at least one layer extending from the end of the tubular enclosure beyond the tip of the electrode a distance of no more than 5 mm from the front tip of the electrode toward the central part of the tubular enclosure. 